Method and apparatus for non-invasive measurement of blood sugar level

ABSTRACT

To intensity-modulate laser light periodically wavelength-modulated by and emitted from a wavelength-variable semiconductor laser 11. To separate the laser light into optical paths 13a, 13b with a beam splitter 14 to irradiate an examined location 17 for assessing blood sugar through path 13a. To detect the intensity of transmitted or reflected light from examined location 17 with a first detector 21 and the intensity of laser light passing through path 13b with a second detector 22 to detect the ratio of the former intensity to the latter intensity with a logarithmic ratio amplifier 25. To detect the rate of change in the ratio with respect to the change in wavelength of the wavelength modulation with a lock-in amplifier 26 to obtain a derivative spectral signal of the absorption spectrum of glucose. An arithmetic processor 27 detects blood sugar in the examined location from the derivative spectrum.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method and apparatus fornon-invasively measuring a blood sugar level on an in vivo and in situbasis using spectroscopic techniques. More specifically, the inventionrelates to a method and apparatus for non-invasively measuring theconcentration of glucose in the blood stream or tissue of a patientsuspected of suffering from diabetes based on a combination ofwavelength modulation and intensity modulation of light.

2. Description of the Related Art

Various methods and apparatus for measuring the concentration of glucosein vitro and in vivo using spectroscopic techniques have been proposed.

For example, International application No. WO 81/00,622 discloses amethod and apparatus for measuring the absorption of infrared light byglucose in body fluid using CO₂ laser light as an irradiation lightsource. The method and apparatus measure the absorption spectra of serumand urine by transmittance and reflectance, i.e.--back scatteringeffects, at different wavelengths λ₁ and λ₂. Here, λ₂ is acharacteristic absorption wavelength of the substance to be measured,e.g. glucose, and λ₁ is a wavelength at which absorption is independentof the concentration of the substance to be measured. The measurementsare obtained by calculating the ratio of the absorbance at λ₁ to theabsorbance at λ₂. The absorption band of the substance to be measured isbetween 940 cm⁻¹ : and 950 cm⁻¹ ; ie. between 10.64 and 10.54 μm forwavelength λ₁, and the absorption band is between 1090 cm⁻¹ ; and 1095cm⁻¹ ; i.e.--between 9.17 μm and 9.13 μm for wavelength λ₂.

U.S. Pat. No. 4,169,676 discloses an non-invasive examining method fordetecting biological substances through skin using anattenuated-total-reflectance (ATR) prism. The method attaches the waveguide (ATR prism) directly to the surface of a sample under examination(e.g. a lip or tongue) and guides in infrared light. The refractiveindex of the wave guide is greater than that of the sample medium, ie.an optically thin layer of the surface, and the infrared light is madeto pass through the prism along the total-reflection path. The infraredlight interacts with the thin layer of the surface, and the interactionis related to the frustrated attenuation component of the light at thereflecting part (see Hormone & Metabolic Res. Suppl. Ser. (1979) pp. 30-35). If infrared light of a wavelength related to the absorption ofglucose is used, then the light passing through the prism is attenuateddepending on the concentration of glucose in the optically thin layer ofthe surface. Therefore, the attenuated quantity is detected andprocessed into data on the glucose concentration.

U.S. Pat. No. 3,958,560 discloses a non-invasive detection apparatusthat detects glucose in a patient's eye. Specifically, the apparatus ofthis U.S. patent is a sensor apparatus in shape of a contact lenscomprising a light source that applies infrared light to one side ofcornea and a detector that detects the transmitted light on the oppositeside. When infrared light is applied to a measured location, theinfrared light passes through the cornea and the aqueous humor andreaches the detector. The detector converts the quantity of transmittedlight into an electric signal and provides it to a remote receiver. Thenthe reader of the receiver outputs the concentration of glucose in thepatient's eye as a function of the individual change of quantity in theapplied infrared light passing through the eye.

British Pat. application No. 2,035,557 discloses a detecting apparatusfor assessing substances near the blood stream of a patient such as CO₂,oxygen, or glucose. The detecting apparatus comprises an optical sourceand an optical receiving means that detects attenuated lightback-scattered or reflected from inside a patient's body, i.e.--from thehypoderma, and uses ultraviolet or infrared light as the irradiationlight.

On the other hand, there are following apparatus that measure or monitorthe flow of blood and organism-activating parameters or components suchas oxygenated hemoglobin and reduced oxyhemoglobin.

U.S. Pat. No. 3,638,640 discloses a method and apparatus for measuringoxygen and other substances in blood and the tissue. The U.S. Pat.apparatus comprises an irradiation light source and a detector placed ona patient's body. If the detector is placed on an ear, then theintensity of light passing through the ear is measured, and if thedetector is placed on a forehead, then the intensity of light reflectedafter passing through blood and the hypoderma is measured. Thewavelengths between red light and near-infrared light are used as theirradiation light, i.e.--660 nm, 715 nm, and 805 nm. The number ofwavelengths used at the same time is 1 plus the number of wavelengthscharacteristic of substances existing in the examined location. Signalsobtained by detecting from absorption at various wavelengths areprocessed by an electric circuit, so that quantitative data concerningthe concentration of the substance to be measured is obtained withoutbeing influenced by the fluctuation of measuring conditions such as thefluctuation of the detector, the deviation of the intensity, thedirection and angle of irradiation, and the fluctuation of the flow ofblood in the examined location.

Further, British pat. No. 2,075,668 discloses a spectrophotometricapparatus for measuring and monitoring metabolic functions of anorganism such as changes in oxidation and reduction of hemoglobins andcytochromes or changes in the blood flow in an organ such as the brain,heart, lever on an in vivo and in situ basis. The apparatus uses anirradiation light of wavelengths between 700 nm and 1,300 nm, whicheffectively penetrates several mm deep under skin.

FIG. 14 of the British patent application illustrates an apparatus formeasuring reflectance comprising a wave guide (optical fiber tube) to beabutted to an organism and a light source. The wave guide is abutted toan organism so that irradiation light is applied to the surface of skinin an oblique direction, and the oriented irradiation light is made topenetrate into the body through skin and to be reflected orback-scattered from the tissue at a distance apart from the lightsource. Some of the light energy is absorbed and the rest is incident ona first detector placed on skin and apart from the light source. Also asecond detector is placed and detects a backward-radiated referencesignal. The analytical signal from the first detector and the referencesignal from the second detector are output into an arithmetic operationcircuit, and the data of analytical information is obtained as theoutput of the arithmetic operation circuit.

In measurement of the concentration of glucose and the like describedabove, the quality of the spectroscopic data obtained by a near-infraredspectrometer is determined by the performance of hardware constitutingthe near-infrared spectrometer. At present, the signal to noise ratioS/N of the best performance is approximately on the order between 10⁵ to10⁶. On the other hand, for example, the prior methods of measuring theabsolute intensity of the spectrum requires 10⁵ to 10⁶ order as the S/Nratio of the spectral signal to measure 100 mg/dL, which is thephysiological concentration of glucose in blood, with spectroscopicallypractical precision, so that the measurement must be done near themaximum precision limit attainable by the spectrometer.

Therefore, methods of measuring the concentrations of sugar and glucoseand the like using spectroscopic techniques have less sensitivity,precision and accuracy than chemical analysis that analyzes theconcentrations of these substances using reagents, and a near-infraredspectrometer of high performance having a high S/N ratio is made withcomplex construction at great cost. Thus, if a variation of glucoseconcentration from the physiological concentration of glucose, 100mg/dL, can be measured with the precision of 2 to 3 digits by areference method, instead of simply measuring the absolute intensity ofa spectrum, then we can find how much the blood sugar of a patientdeviates from a normative value, so that the measurement can befavorably used for controlling the blood sugar of the patient.

SUMMARY OF THE INVENTION

The object of the present invention is therefore to provide a method ofmeasuring blood sugar that easily and non-invasively measures thevariation of the blood sugar of a patient suspected of suffering fromdiabetes from a normative value independent of the patient's individualdifferences using a modulation means that combines wavelength modulationwith intensity modulation.

Another object of the present invention is to provide a compact andinexpensive apparatus for measuring blood sugar that easily andnon-invasively measures the variation of the blood sugar of a patientsuspected of suffering from diabetes from a normative value independentof the patient's individual differences with a simple constructioncomprising a wavelength modulation means and an intensity modulationmeans.

In order to achieve the aforementioned objectives, the present inventionintensity-modulates light with several intensities as well aswavelength-modulating, applies the modulated light to an examinedlocation for assessing blood sugar, detects, for eachintensity-modulated light, the intensity of the transmitted andreflected light from a portion to be examined and the intensity of theincident light onto the portion to be examined, detects the ratio of thetwo intensities, detects the rate of change in the ratio with respect tothe change in the wavelength due to the above wavelength-modulation,extracts the derivative spectrum of the absorption spectrum of glucosein that portion, and detects the blood sugar of that portion based onthese derivative spectra for all modulating intensities of light.

In this way, light which is intensity-modulated as well as beingwavelength-modulated with a small modulation width Δλ around aconsidered wavelength is applied to the examined portion, and the depthof penetration into skin is varied by the intensity modulation of theincident light on the examined location, so that information concerningthe concentration of glucose in the examined portion, where body fluidincluding blood components exists, is extracted, and determination ofglucose in the examined location is performed based on the derivativespectra of the absorption spectra. Therefore, the concentration ofglucose is easily and securely detected independently of individualdifferences of the patient.

The above derivative spectra are preferably accumulated and averagedcorresponding to the iteration of the above wavelength modulation. If,in this way, the derivative spectra are accumulated and averaged, thenthe noise component is reduced in proportion to the square root of thenumber of accumulation, so that the signal to noise ratio S/N isimproved.

The present invention provides an apparatus comprising awavelength-modulated light generator that generates wavelength-modulatedlight, an intensity modulator that intensity-modulates thewavelength-modulated light output from the wavelength-modulated lightgenerator into several intensities, a beam splitter that separates theoptical path of the wavelength-modulated and intensity modulated lightemitted from the intensity modulator, an optical collector that collectsthe light passing along one of the optical paths separated by the beamsplitter, made incident on the examined location for assessing bloodsugar, and being transmitted or reflected thereby, a first photodetector that detects the intensity of the light collected by theoptical collector, a second photo detector that detects the intensity ofthe light passing along the other path separated by the beam splitter, aratio detector that detects the ratio of the output of the first photodetector to the output of the second photo detector, a derivativespectral signal detector that reads a ratio signal output from the ratiodetector, detects the rate of change in the ratio signal with respect tothe change in wavelength due to the above wavelength modulation, anddetects the derivative spectral signal of the absorption spectrum ofglucose in the examined portion, an arithmetic means that calculatesblood sugar in the examined portion for each intensity of theintensity-modulated light based on the derivative spectral signaldetected by the derivative spectral signal detector.

The present invention wavelength-modulates light in thewavelength-modulated light generator and intensity-modulates thewavelength-modulated light to make it incident on the examined portionand detects the difference spectrum of the absorption spectrum ofglucose, so that derivative data of high quality is obtained in realtime without requiring computer processing. Further, the speed ofiterative scanning is higher than an ordinary spectrometer, which scansa wide range of wavelengths, so that measured data on the concentrationof glucose can be obtained by short-time photometry without being muchinfluenced by a drift of the optical system.

The above wavelength-modulated light generator is preferably awavelength-variable semiconductor laser. A semiconductor laser developedfor use in optical fiber communications can be employed as thewavelength-variable semiconductor laser, so that the characteristics ofa wavelength-variable semiconductor laser can be effectively utilized atits maximum performance, and the construction of the means forwavelength-modulating the measured light is extremely simplified.Therefore, the construction of the apparatus for non-invasivemeasurement of blood sugar becomes simple and compact.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention willbecome clear from the following description taken in conjunction withthe preferred embodiments thereof with reference to the accompanyingdrawings throughout which like parts are designated by like referencenumerals, and in which:

FIG. 1 shows a single peak spectrum, its first derivative spectrum, andits second derivative spectrum.

FIG. 2 shows the generation of a derivative spectrum bywavelength-modulation spectroscopy.

FIG. 3 shows an absorbance spectrum of an aqueous solution of glucose.

FIG. 4 shows the first derivative spectrum of FIG. 3.

FIG. 5 shows the difference absorbance spectra with respect to standardpure water.

FIG. 6 shows the difference of the first derivative spectra.

FIG. 7. shows the difference of the first derivative spectra.

FIG. 8 shows the difference of the first derivative spectra.

FIG. 9 shows the difference of the first derivative spectra.

FIG. 10 shows the relationship between the concentration of glucose andthe first derivative of an absorbance spectrum.

FIG. 11 shows the structure of human skin for describing its opticalproperties.

FIG. 12 shows a graph for describing the relationship between theintensity of incident light and the depth of light penetration.

FIG. 13 shows a block diagram of an apparatus for non-invasivemeasurement of blood sugar.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments according to the present invention will bedescribed below with reference to the appended drawings.

Items [1] and [2] below describe the derivative spectroscopy necessaryfor understanding the present invention and a method of wavelengthmodulation for obtaining derivative spectra. Furthermore, items [3],[4], and [5] respectively described the verification of determiningglucose from the first derivative spectra, the choice of optimalwavelength, and the diffuse reflectance spectra of skin andintensity-modulation spectroscopy. Finally the configuration of anapparatus for non-invasive measurement of blood sugar in accordance withthe present invention is described in item [6].

[1] Derivative spectroscopy

Wavelength modulation is generally used to obtain derivative spectra.The method of wavelength modulation is described in T. C. O'Haver,"Potential clinical applications of derivative and wavelength-modulationspectroscopy", (Clinical Chemistry, Vol 25, No. 9 (1979), pp.1548-1553). The concept of wavelength-modulation spectroscopy is closelyconnected to the concept of derivative spectroscopy, and they are bothbased on the measurements of changes in intensity and absorbance withrespect to a change in wavelength.

First, derivative spectroscopy is described. Derivative spectroscopyobtains the first or higher-order derivatives of the intensity orabsorbance spectrum with respect to wavelength and plots the results.The purposes of the derivative spectroscopy are:

(a) the compensation and correction of the baseline shift, and

(b) the effective increase in sensitivity to subtle changes in the shapeof the spectral band.

FIG. 1 shows a single peak spectrum and its first and second derivativespectra. The peak maximum point P_(max) corresponds to the zero-crossingpoint P₀₁ of the first derivative and the central peak point P_(c) ofthe second derivative. The peak maximum point P_(dmax) and the peakminimum point P_(dmin) of the second derivative respectively correspondto the maximum slope points P_(s1) and P_(s2) of the original spectrumand also respectively correspond to the zero-crossing points P₀₂ and P₀₃of the second derivative.

There are several methods of obtaining derivative spectra as follows.

First, if the spectral data are digital values and can be processed by acomputer, then the derivative spectra can be computed by numericaldifferentiation in software.

Secondly, the derivative spectra can be acquired in real time throughtime derivatives obtained by constant-speed scanning in hardware. Thistechnique is based on the fact that if the wavelength scanning ratedλ/dt is constant, then the derivative dI/dλ of the intensity I withrespect to wavelength λ is proportional to the derivative dI/dt of theintensity I with respect to time t, as is clear from the following (1).That is, by means of an electronic differentiator, the followingequation (1) can be calculated.

    dI/dλ=(dI/dt)/(dλ/dt)                        (1)

Thirdly, derivative spectra can be obtained by a wavelength modulationdescribed below.

As shown in FIG. 2, a technique of wavelength modulation irradiates asample with periodically modulated light having a narrow modulationwidth Δλ around a particular wavelength λ_(i) and detects thetransmitted or reflected light with a detector. The ripple or thealternating-current component of the output signal from the detector isseparated or electrically measured. If the modulation width Δλ issufficiently smaller than the bandwidth of the spectrum, then thealternating-current component of the optoelectronic signal at themodulation frequency generates an alternating-current signal, i.e.--aderivative spectrum D, which has an amplitude proportional to the slopeof the spectrum within the modulation wavelength width.

There are several techniques for the wavelength modulation describedabove as follows:

(a) vibrating the slit, mirror, diffraction grating, or prism of amonochromator.

(b) inserting a vibrating mirror or rotary refracting mirror.

(c) using a wavelength-continuous-variable filter.

(d) vibrating or tilting a diffraction filter.

(e) vibrating a Fabry-Perot interferometer. Besides,

(f) using a continuous-wavelength-variable semiconductor laser can bealso considered.

The method of installing a reflective diffraction grating outside asemiconductor laser and controlling the angle of the diffraction gratingto vary the oscillatory wavelength has been known. This method can varythe wavelength in a narrow spectral line width. If the variation is notnecessarily continuous and if jumps between longitudinal modes areallowed, then the construction of the apparatus can be simplified.

If a single-mode filter that is synchronous with a tuning wavelengthwithin a narrow bandwidth is added, then oscillation occurs at anarbitrarily set wavelength in a single mode. This apparatus is called atunable semiconductor laser of the external resonance type.

Furthermore, a wavelength-variable semiconductor laser developed for useof coherent optical communications is described in Nikkei electronics,No. 423 (Jun. 15, 1987), pp. 149-161. In this article, semiconductorlasers that control wavelength with a tri-electrode construction basedon the distributive Bragg-reflection laser of single mode are described.One of the semiconductor lasers continuously varies wavelength in asingle longitudinal mode within a wavelength range of 3.1 nm. If thelongitudinal mode is allowed to change in a middle, then the wavelengthrange is about 6 nm.

[2] Method of wavelength modulation for obtaining derivative spectra.

If, in wavelength modulation, the modulation width Δλ(=λ₂ -λ₁) issufficiently less than the bandwidth of the spectrum, then thealternating-current component of the optoelectronic signal at themodulation frequency generates an alternating-current signal ΔI/Δλ,i.e.--a derivative spectrum D expressed by the following (2), which hasan amplitude proportional to the slope of the spectrum within themodulation wavelength width. The amplitude of the alternating-currentsignal can be obtained in real time by an appropriate electrical system.

    D=ΔI/Δλ=(I.sub.2 -I.sub.1)/(λ.sub.2 -λ.sub.1)                                          (2)

In general, the direct-current component is greater than thealternating-current component in measurement of a low concentration ofglucose. Since the direct-current-component having such insignificantgreat values can be cut off, the dynamic range of the A-D converter usedin an apparatus for the measurement of blood sugar described later canbe efficiently used, and mathematical processing is performed thereafterat an advantage.

Wavelength modulation is performed by scanning periodically upward anddownward within a narrow modulation width Δλ, so that the scanning canbe repeated at a higher speed than by an ordinary spectrometer, whichscans a wide range of wavelength. Therefore, the accumulation andaveraging are easily performed. Since the noise component can be reducedin proportion to the square root of the number of accumulatedmeasurements, the signal to noise ratio (S/N) can be improved by makingthe number of accumulated measurements large. Furthermore, shortmeasurement times effectively suppress a drift of the optical system ofthe spectrometer.

The wavelength range in wavelength modulation is limited to a narrow Δλ,but derivative spectra of a high quality are obtained in real timewithout any computer processing. Therefore, wavelength modulation issuitable for a routine analysis of samples whose characteristics arealready well known, for example, for quality control and clinicalanalysis.

On the other hand, if an original spectrum of digital values isprocessed by a numerical derivative operation, the numerical precisionand quality of the intensity I_(i) itself pose a problem.

The process of obtaining a derivative spectrum tends to enhancehigh-frequency noise in the original spectrum. If used improperly, theS/N ratio is greatly reduced by a derivative operation of a spectrum oflow quality.

Further, in measurement of a sample of low absorption, unless thenumerical precision or the number of significant digits of the intensityI_(i) of an original spectrum is great, a significant change in thedesired derivative spectrum can not be obtained. That is, the S/N rationeeds to be very large.

[3] The verification of determining glucose from first derivativespectra

If the spectral data have digital values, then their derivative spectracan be computed by numerical differentiation of the absorbance spectra.Therefore, we obtained the first derivative spectrum of an absorbancespectrum obtained by a Fourier-transform spectrometer by numericaldifferentiation to test the validity of the determination of glucoseconcentration by the wavelength modulation technique.

As samples, and we used pure water, aqueous solutions of glucose of1,000 mg/dL, 3,000 mg/dL, and 5,000 mg/dL.

Since it is difficult to observe the differences among samples in detailin comparing the absorbance spectrum and the first derivative spectrumof each sample with that of each other sample, we calculated thedifferences between each sample and the standard pure water. That is, wecalculated the difference absorbance spectrum and the difference of thefirst derivative spectrum of each sample to make the differencesobservable. The derivative operation was performed in the direction fromlonger wavelength to shorter wavelength.

First, let us consider the glucose absorption band between theabsorption peaks 1.43 μm and 1.93 μm of pure water. FIG. 3 shows theabsorbance spectrum, and FIG. 4 shows its first derivative spectrum.Further, FIG. 5 shows the difference absorbance spectra. In thedifference absorbance spectra of FIG. 5, the absorption by glucose isobserved between 1.55 μm and 1.85 μm. Also, S-shaped characteristics areobserved between 1.35 μm and 1.45 μm. These are due to the shift of theabsorbance peak 1.43 μm of pure water caused by hydration. The centralwavelength of the wavelength modulation can be chosen from thewavelength ranges, one between 1.45 μm and 1.58 μm, which is around thenoninterference zero-crossing point, one between 1.6 μm and 1.67 μm, andone between 1.75 μm and 1.85 μm, which are less affected by interferenceand have steep slopes in an absorption band.

As is observed from the difference of the first derivative spectra shownin FIG. 6, it is clear that glucose can be determined by the firstderivative spectrum. FIG. 10 shows the relationship between the firstderivative of the absorbance and the glucose concentration at wavelength1.555 μm.

Since wavelength-variable semiconductor lasers can be employed for the1.5-μm band, the construction of the apparatus is easy. Ifwavelength-variable semiconductor lasers are applied to wavelengthmodulation, the characteristics of wavelength-variable semiconductorlasers can be effectively used to the maximum performance limit.

Beyond the absorption peak 1.93 μm of pure water, there are absorptionbands of glucose at 2.1 μm, 2.27 μm, and 2.33 μm. The slopes aroundthese absorption peaks should be considered carefully. As is observedfrom the derivatives of difference absorbance spectra shown in FIG. 7,the central wavelength can also be chosen from

2.06˜2.1 μm

2.1˜2.24 μm

2.24˜2.27 μm

2.27˜2.3 μm

2.3˜2.32 μm

2.32˜2.38 μm

Similarly, between the absorption peaks 0.96 μm and 1.15 μm of purewater, there is a broad absorption band of glucose at 1.06 μm. As isobserved from the difference of the first derivative spectra shown inFIG. 8. The central wavelength can be chosen from the range between 1.07μm and 1.25 μm and the range between 1.00 μm and 1.05 μm.

Similarly, between the absorption peaks 1.15 μm and 1.43 μm of purewater, there is a broad absorption band of glucose at 1.25 μm. As isobserved from the difference of the first derivative spectra shown inFIG. 9, the central wavelength can be chosen from the range between 1.28μm and 1.36 μm and the range between 1.18 μm and 1.23 μm.

[4] Choice of optimal wavelength,

Human skin consists of the cornified layer 1, epidermis 2, and dermis 3successively from the outside, as shown in FIG. 11, and has ananisotropic structure in the direction of depth. In measuring theconcentration of glucose in the part in which body fluid containingblood components exists, i.e.--the capillary bed 4, by means of diffusereflection through skin, the wavelength selection is important andinseparable from the method of measuring.

A longer-wavelength region near middle-infrared light and ashorter-wavelength region near visible light in the near-infrared regionare compared in the following.

In the longer-wavelength region, light energy is absorbed strongly bywater existing in the organism, so that it is hard to penetrate into adeeper part of the organism (skin). However, light is hard to beattenuated since it is less affected by scattering. Also, since theabsorption coefficient of glucose in its existing absorption band isgreater, the path length can be short, ie. the depth of lightpenetration can be relatively small.

In the shorter-wavelength region near visible light, light is lessabsorbed by water to reach a deep part of skin. However, light is easilyaffected and attenuated by scattering. Also, since the absorptioncoefficients of glucose in its absorption band are small, the pathlength must be large to raise the sensitivity of measurement.

In this way, there are various related factors for choosing optimalwavelength. In conclusion, an optimal wavelength for measurement ofglucose is preferably chosen from the range between 1.45μ and 1.85 μmbecause of the chosen wavelength band described in [3] and thecharacteristic absorption coefficients of glucose, the depth of lightpenetrating skin, and a practical factor. The practical factor means thefact that a wavelength-variable semiconductor laser for coherent opticalfiber communications can be employed.

[5] Diffuse reflectance spectra of skin and intensity-modulationspectroscopy

As described earlier, derivative data of high quality are obtained inreal time by wavelength modulation without requiring computerprocessing. Derivative data are, in a way, data at one point, so that,from a practical standpoint, it is important that data are normalizedand that various fluctuating factors such as changes in the temperatureof the sample and interaction of chemical components are automaticallycompensated. The present invention combines wavelength modulation withintensity modulation to automatically compensate for these fluctuatingfactors.

A diffuse reflectance spectrum of skin is based on a signal obtainedfrom the weak diffuse reflection of light which has been repeatedlyabsorbed and scattered inside skin and collected by an integratingsphere and detected by a detector. In relation to the anisotropicstructure in the direction of depth, the diffuse reflectance spectrum isa mixture spectrum comprising the following spectral components of theincident light 5:

(a) Spectral components of regularly reflected light 7 on the surface ofskin.

(b) Spectral components of diffuse-reflected light 8 from the cornifiedlayer 1 or surface tissue that does not contain glucose.

(c) Spectral components of diffuse-reflected light 9 from the part 4where body fluid containing blood components exist.

(d) Spectrum components of transmitted light through deeper tissue.

In general, the contribution of spectral components near the surface ofskin is great, and the contribution of spectral components in part 4where body fluid containing blood components exist is small. This factcharacterizes an ordinary diffuse reflectance spectrum.

If we are concerned with the concentration of glucose in part 4 wherebody fluid containing blood components exists, and if we can determineand analyze a spectrum not containing the spectral components of theabove (a) and (b), then clearly we can measure the concentration ofglucose more accurately.

As a technique to realize this possibility, the inventors of the presentapplication proposed a following technique of light-intensity modulationin Japanese Patent Application No. Sho-62-290821 and U.S. Pat. No.4,883,953.

The technique controls the depth of light penetration by varying theintensity of incident light. As shown in FIG. 12, when the intensity ofincident light is great, then more information of greater depth isincluded than when the intensity is small. Therefore, the incident lightof intensity I₀₁ at which penetration depth for a detection limit is b₁is used, and the intensity I_(s1) of diffuse-reflected light from depthb₁ /2 is measured. Then the ratio between them is calculated by thefollowing equation (3) for normalization.

    A.sub.1 =log (I.sub.01 /I.sub.s1)                          (3)

A₁ has spectral information of only the part near the surface of skin.

Next, the incident light of intensity I₀₂ in which penetration depth fordetection limit is b₂, which is greater than b2, is used, and theintensity I_(s2) of diffuser-reflected light from depth b₂ /2 ismeasured. Then the ratio between them is calculated by the followingequation (4) for normalization.

    A.sub.2 =log (I.sub.02 /I.sub.s2)                          (4)

A₂ contains spectral information of deeper part from the surface ofskin. Then the difference ΔA between A₁ and A₂ is calculated.

    ΔA=A.sub.2 -A.sub.1 =log (I.sub.02 /I.sub.s2)-log (I.sub.01 /I.sub.s1)(5)

The ΔA of the above equation (5) expresses spectral information from thebaseline spectrum of an examined subject's tissue near the surface ofthe skin in which no glucose is contained. Therefore, ΔA is free fromthe influence of the subject's individual differences such as race, sex,and age.

The modulation of incident light can be performed by switchingattenuators having different attenuation ratios by a rotating disk. Theabsorbances are normalized by calculating the above ratios (3) and (4)for each cycle of the modulation of the intensity of incident light, andthe difference of the normalized absorbances is calculated by (5). Thenthe differences are accumulated and averaged for many cycles to improvethe S/N ratio.

A regression equation is created using the averaged differences forsamples having different concentrations of glucose and referenceconcentration values obtained by chemical analysis. Finally, using thisregression equation, glucose of an unknown sample is determined.

We have described the algorithm of the technique of intensity modulationof incident light using the spectral intensity I. It is known by themethod of regression that determinacy also exists between the derivativeintensity and the concentrations. Therefore, in order to use the firstderivative D=ΔA/Δλ, we replace the absorbance A in equations (3), (4),and (5) with ΔA/Δλ to obtain the equations (8), (9), and (10) describedlater.

[6] apparatus for non-invasive measurement of blood sugar

FIG. 13 shows a block diagram of an apparatus for non-invasivemeasurement of blood sugar in accordance with the present invention.

The above apparatus for non-invasive measurement of blood sugar has asits components a wavelength-variable semiconductor laser 11, anattenuator 12 that periodically varies the intensity ofwavelength-modulated laser light output from semiconductor laser 11, abeam splitter 14 that separates the optical path 13 of thewavelength-modulated and intensity-modulated light emitted fromattenuator 12 into an optical path 13a and an optical path 13b, and anintegrating sphere 18 that collects laser light transmitted or reflectedafter passing along optical path 13a and made incident on an examinedportion 17 of skin 16 in which the blood sugar level is measured.

The above apparatus for non-invasive measurement of blood sugar levelsalso has as its components a first detector 21 that detects theintensity of the laser light collected by the integrating sphere 18, asecond detector 22 that detects the intensity of laser light passingalong optical path 13b, an amplifier 23 that amplifies the output of thefirst detector 21, an amplifier 24 that amplifies the output of thesecond detector 22, a logarithmic ratio amplifier 25 that outputs thelogarithm of the ratio between the outputs of amplifiers 23 and 24, alock-in amplifier 26 that detects the derivative spectral signal of aglucose absorbance spectrum in above examined portion 17 from the rateof change of the output of logarithmic ratio amplifier 25 with respectto a change in wavelength, an arithmetic processor 27 containing amicroprocessor that calculates the blood sugar level in the aboveexamined portion by processing a derivative spectral signal, which is adigital signal obtained by converting the above analog derivativespectral signal detected by the lock-in amplifier 26.

Laser light adjusted and controlled at the central wavelength λ_(i) andthe wavelength-modulation width Δλ by wavelength-variable semiconductorlaser 11 is separated into two beams by beam splitter 14 after beingintensity-modulated by attenuator 12.

One laser beam L₂ passing through beam splitter 14 is converted into anelectrical signal I₀ by second detector 22, so that the intensity of theincident light is monitored. The other laser beam L₁ is made incident onexamined location 17 where the concentration of glucose is measured. Thelight diffuse-reflected from examined location 17 is converted into anelectrical signal I_(s) by the first detector 21 after being collectedby integrating sphere 18.

The above electrical signals I_(s) and I₀ are respectively amplified bythe amplifiers 23 and 24, and is input to logarithmic ratio amplifier25, which outputs the normalized absorbance signal expressed by thefollowing equation (6).

    A=log (I.sub.0 /I.sub.s)                                   (6)

Since the above electric signals I_(s) and I₀ are values measured at thesame time by the first detector 21 and the second detector 22 after thesame laser light is separated by the beam splitter 14, the values of theabove absorbance signal A are accurate and are hardly affected by drift.

Then only the amplitude of an alternating-current signal expressed bythe following equation (7) is extracted by the lock-in amplifier 25.

    D=ΔA/Δλ                                 (7)

The alternating-current component is a signal proportionate to the slopeof the spectrum of a sample at the central wavelength of the wavelengthmodulation.

As described earlier, the attenuator 12 varies the intensity of theincident light to vary the depth of light penetration into examinedlocation 17 of skin 16 and switches two attenuator units 12a and 12b ormore than those by a rotating disk 12c. The concentration of glucose ina part where body fluid containing blood components exists is measuredaccurately by the variation of the intensity of the incident light.

Lock-in amplifier 26 outputs an alternating-current signal expressed bythe following equation (8) corresponding to the intensity I₀₁ of theincident light created by attenuator 12.

    D.sub.1 =ΔA.sub.1 /Δλ                   (8)

Lock-in amplifier 26 also outputs an alternating-current signalexpressed by the following equation (9) corresponding to the intensityI₀₂ of the incident light created by attenuator 12.

    D.sub.2 =ΔA.sub.2 /Δλ                   (9)

Arithmetic processor 27 converts the above alternating-current signalsD₁ and D₂ from analog to digital format and calculates the differenceexpressed by the following equation (10) for each cycle of the intensitymodulation of the incident light.

    ΔD=D.sub.2 -D.sub.1 =ΔA.sub.2 /Δλ-ΔA.sub.1 /Δλ                                          (10)

Arithmetic processor 27 uses the values obtained by equation (10) andthe data of a regression equation, which is obtained beforehand and notillustrated in FIG. 13, to determine the glucose concentration level inthe examined location.

In the above determination of the glucose concentration level, if theprocessing of accumulation and averaging is performed for many cycles ofthe switching of attenuator units 12a and 12b of attenuator 12, then theS/N ratio is improved.

Further, if the incident light is intensity-modulated at more than 3steps, then an optimal range of intensities of the incident light isfound for the determination of the glucose concentration level, so thatan optimal choice of attenuator 12 can be made, and the accuracy of thepresent technique is further enhanced. As a result, a standard diffuseplate used for calibration in prior diffuse reflectance methods becomesunnecessary.

Further, if light penetrating examined portion 17 does not leak from thebottom of the sample, ie. the condition of the so-called infinitethickness of the sample, is satisfied, then the information on thethickness of the examined location is not necessary unlike thetransmission method.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as being included within the scope ofthe present invention as defined by the appended claims unless theydepart therefrom.

What is claimed is:
 1. A method for non-invasive measurement of bloodsugar levels comprising the steps of:providing light which isintensity-modulated with a plurality of intensities as well as beingwavelength-modulated; applying the modulated light to an examinedportion; detecting an intensity of reflected light from said examinedportion and an intensity of the incident light onto said examinedportion for each intensity of said modulated light; detecting the ratioof an intensity of said reflected light and said incident light;detecting the rate of change in said ratio with respect to the change inwavelength due to the wavelength modulation; extracting a derivativespectrum of an absorbance spectrum of glucose in said examined portion,and detecting the blood sugar level of said examined portion.
 2. Themethod for non-invasive measurement of blood sugar levels as defined inclaim 1, including extracting said derivative spectrum by accumulatingand averaging in correspondence with the iteration of said wavelengthmodulation.
 3. An apparatus for non-invasive measurement of blood sugarlevels comprising:a wavelength-modulated light generator for generatinga wavelength-modulated light; an intensity-modulator forintensity-modulating the wavelength-modulated light output from saidwavelength-modulated light generator with a plurality of intensities; abeam splitter for separating the optical path of thewavelength-modulated and intensity-modulated light emitted from saidintensity modulator into two optical paths; an optical collector forcollecting the light passing along one of the two optical pathsseparated by said beam splitter, being incident on an examined locationfor assessing the blood sugar level, and being reflected therefrom; afirst optical detector for detecting an intensity of the light collectedby said optical collector; a second optical detector for detecting anintensity of the light passing along the other optical path separated bysaid beam splitter; a ratio detector for detecting the ratio of theoutput of said first optical detector to the output of said secondoptical detector; a derivative spectral signal detector for receiving aratio signal from said ratio detector and for detecting the rate ofchange in said ratio signal with respect to the change in wavelength dueto the wavelength modulation to detect a derivative spectral signal ofan absorbance spectrum of glucose in said examined portion; anarithmetic means for calculating the blood sugar level in said examinedlocation based on the derivative spectral signal detected by saidderivative spectral signal detector.
 4. The apparatus for non-invasivemeasurement of blood sugar levels as defined in claim 3, wherein saidwavelength-modulated light generator is a wavelength-variablesemiconductor laser.
 5. An apparatus for non-invasive measurement ofblood sugar levels comprising:a wavelength-variable light source forwavelength-modulated light; an attenuator for periodically varying theintensity of the light output from the light source; a beam splitter forseparating light output from the attenuator into a reference light beamand a measuring light beam; an integrating sphere for collecting lightreflected from an examined portion of skin in which the blood sugarlevel is to be measured, the reflected light caused by the measuringlight beam being projected on the examined portion of skin; a firstdetector for detecting the intensity of the light collected by theintegrating sphere; a second detector for detecting the intensity of thereference light beam; first and second amplifiers for respectivelyamplifying outputs of said first and second detectors; a logarithmicratio amplifier for receiving outputs from said first and secondamplifiers and for outputting a logarithm of a ratio between outputs ofthe first and second amplifiers; a lock-in amplifier for receiving anoutput from said logarithmic ratio amplifier and for detecting aderivative spectral signal of a glucose absorbance spectrum for theexamined portion from a rate of change of the output of the logarithmicratio amplifier with respect to a change in wavelength; a processingmeans for receiving an output from said lock-in amplifier and forcalculating a blood sugar level in the examined portion by processingthe derivative spectral signal detected by said lock-in amplifier.